The present invention relates to methods of measuring acoustic signals, such as those produced by a person's pulse or voice, by using wireless signals, and in particular to a system using an antenna array for practical vibrometry in situations with small vibrating targets and/or weak signals.
Devices such as wrist mounted fitness monitors may monitor pulse by measuring changes in reflected light caused by blood flow (photoplethysmograhy). In each cardiac cycle, a pressure pulse distends the arteries slightly increasing reflective area of the blood. Accurate readings using photoplethysmograhy often requires repositioning the measuring device from the wrist to the forearm and securing it tightly about the forearm. Cold weather, tattoos, and irregular movements of the arm may interfere with measurements. Recently there has been some concern that photoplethysmography techniques can be inaccurate at high intensity workout levels.